Autoimmune diseases are among the leading causes of death and disability in the United States and other developed countries. Autoimmune disease occurs when the immune system mistakenly unleashes its arsenal of defenses upon normal tissues. This response may be inadvertently triggered during an immune response to infection by microbes. The mechanisms for shutting down immune responses to microbial invaders are not understood. These regulatory mechanisms may play an important role in preventing autoimmune disease in healthy individuals. Thus, a greater understanding of mechanisms to downregulate immune responses may lead to better therapies for autoimmune diseases. Recently we have shown that the cytokine IFN-gamma and nitric oxide play important roles in turning off the immune response during infection with Mycobacterium bovis (BCG) and during experimental autoimmune encephalomyelitis (EAE), a mouse model for the autoimmune disease Multiple Sclerosis. Additionally, we and others have found that infection of mice with BCG prevents autoimmune diseases such as autoimmune diabetes of NOD mice and EAE. We have found that the protection of mice from autoimmune disease by mycobacterial infection requires the production of IFN-gamma and nitric oxide by the BCG-infected host. This raises several questions about whether the IFN-gamma-dependent mechanism for turning off Th1 CD4 T cell responses is part of the mechanism for protection of mice from autoimmune disease by BCG infection. We will address several issues in the current application. In Aim 1 we will examine the in vivo fate of Th1 EAE-inducing CD4 T cells after transferring them to uninfected and BCG-infected recipients. The effect of IFN-gamma and nitric oxide on the fate of these cells will also be determined by comparing wild-type, IFN-gamma KO, and nitric oxide-ablated donors and recipients. In Aim 2 we will elucidate the mechanism of IFN-gamma -dependent anergy and apoptosis of bystander CD4 T cells in vitro by BCG-infected splenocytes. Using this model these studies will determine (i) which spleen cells and molecules are required for suppression of CD4 T cells by IFN-?, (ii) whether BCG infected cells must present antigen to suppress CD4 T cells (iii) which soluble molecules mediate suppression (iv) whether Fas or CTLA-4 are involved in suppression and (v) whether the anergic CD4 T cells produce IL-2. This will allow us to relate this mechanism of CD4 T cell downregulation to known mechanisms. In Aim 3 we w ill determine the importance of live versus killed BCG, the route of infection, and the duration of protection against autoimmune disease by BCG infection. These studies will elucidate the mechanism of IFN-gamma -and nitric oxide dependent downregulation of Th1 CD4 cell responses.